1. Field of the Invention
The present invention relates to a field effect transistor for liquid crystal display, and more particularly to an inverted-staggered type field effect transistor with excellent stability for liquid crystal display.
2. Description of the Related Art
In recent years, active matrix type liquid crystal display in which thin-film transistors are used as switching elements occupies an increasingly higher percentage of liquid crystal displays. Thin-film transistors used generally for this purpose are of amorphous silicon type which can be fabricated at low temperatures on a large area glass substrate. Besides, many of such amorphous silicon thin-film transistors (a-SiTFT) have the so-called inverted staggered structure in which source and drain electrodes are formed above a gate electrode.
FIG. 1 illustrates in cross-sectional view an example of a thin-film transistor for liquid crystal display element in the prior art (See Symposium of JAPAN DISPLAY '89, p. 516, FIG. 4).
As shown in FIG. 1, a gate electrode 2 is formed on a glass substrate 1. The gate electrode 2 is made of an alloy of Mo and Ta. The gate electrode 2 and the glass substrate 1 are covered with a deposited gate oxide film 3 of silicon oxide (SiO.sub.2). An undoped amorphous silicon film 4 is selectively formed on the gate oxide film 3 above the gate electrode 2. An n.sup.+ -type amorphous silicon film 5 is formed on the amorphous silicon film 4. Besides, on the gate oxide film 3, a pixel electrode 6 of indium-tin oxide (ITO) is also formed in a selected area except the area just above the gate electrode 2. There is further a pair of source and drain (Mo film 7a and Al film 8) electrodes formed in specified patterns by sequential deposition of Mo film 7a and Al film 8 over the whole resultant surface and then etching away to selectively remove them. The source and drain regions (n.sup.+ amorphous silicon film 5) are patterned by selectively removing the n.sup.+ -type amorphous silicon film 5 between the source and drain electrodes (in the area just above the gate electrode). At the top, there is a passivation film 9 of silicon nitride (SiNx) deposited except the pixel electrode 6. Finally, a liquid crystal 14 is inserted between the glass substrate 1 and an opposite electrode 13 formed on an opposite glass substrate 12 to form a liquid crystal display.
In a thin-film transistor for liquid crystal display having such inverted-staggered structure as described above, the so-called back channel region which is between the source and drain regions 5 and is opposed to the gate electrode is formed with its surface upward, and SiNx film 9 formed thereon as a passivation film works as a barrier to the Na.sup.+ ions.
In the above-described conventional thin-film transistor for liquid crystal display, a problem is encountered that when SiNx film 9 is formed on a nondoped amorphous silicon film 4 containing a large amount of hydrogen, there is a tendency to generate fixed charges on the back-channel side of amorphous silicon film 4, which may cause an increase in an OFF current between the source and drain regions and/or a change in the threshold voltage, resulting in changing the characteristics of the transistor.